Names like orbital sander, pad sander, and oscillating sheet sander are used to identify finishing sanders. You'll find a finishing sander, too, the palm-grip sander, which is a reasonably priced, easy-to-use, efficient tool. Technically speaking, it's an orbital sander, mean-ing that it sands in a rotating pattern.
Some finish sanders function in a different fashion, sanding in a straight-line mode, backwards and forwards, much like hand sanding. Some finishing sanders move from straight-line to orbital sanding and back again at the flick of a switch. Do you need to be able to do both? The consensus among the pros is that straight-line sanding is less likely to leave little whirls of cross-grain scratches, so for the final pass, it's preferred to orbital movements (though if little pressure is applied to an orbital sander using fine paper, the scratches will be minimal).
There are a great range of sizes for such finishing sanders, generally identified by the portion of a standard sheet of sandpaper they require: one-half, one-third, one-quarter, one-fifth, or even one-sixth of a sheet.
Most of these sanders are held in one hand, are driven by an electric motor, and have the proper sized sheet of sandpaper mounted in twin jaws that hold the paper tight to a rubber pad at the base of the unit. (Some have an added handle up front for two-handed work.) All these tools are capable of doing a perfectly acceptable job, though some are faster or slower than others, or easier or harder to use. Some offer only one speed, others two, still others a variable-speed adjustment.
But I've come to favor a different species of finish sander, a clever variation of the familiar orbital sander. The basic design of this sander has been around for years in auto body shops, but is just coming into general use in the woodshop. Called a random-orbit sander, this sophisticated tool is now affordable for just about anybody who is ready to invest in a basic finish sanding machine.
The sandpaper used in a random-orbit sander is shaped like a disk, but it is the action of the machine that makes it truly different from its predecessors. The sanders motor spins a shaft on which a counter-weight is mounted. The combination of the weight, the spinning shaft, and an offset thrust bearing produces a random. Varying motion. To put it less technically, it spins and wiggles, never the same way twice, and does a nice, neat job, producing little or no scratching across the grain. Another advantage of the random-orbit sander is that a great deal of material ran be removed without scratching the finish.
The varying motion also lends to clean the sandpaper, meaning that it lasts longer. Purpose-made sandpaper disks are required: they are sold with backing that is pressure-sensitive (adhesive) has hook and loop (Velcro) fastenings. The latter can be removed from the sander and reused over and over again.
Random-orbit sanders are sold in single-speed and variable-speed models.
Most models come with a dustbag attachment (it's an inexpensive option from some manufacturers). This, too, reduces the frequency with which the dust clogs the paper on the machine, and results in a cleaner work area. I recommend it.
In addition to models powered by electric motors, random-orbit sanders that run on air power are also available. You need a compressor and an air hose to power this variety, but if your workshop is so equipped, this is a quieter and lighter-weight alternative.
Some of the motor-powered random-orbit sanders are in-line models, with the motor mounted vertically (they resemble rounded-off palm sanders). Right-angled models look a bit like a saber saw, in that the motor and its housing are perpendicular to the driveshaft. Right-angled models are more expensive, but since they have more powerful motors and allow for a firmer, two-handed grip, they are also more versatile machines.
To use the sander, switch it on and let it come to full speed. Then set it on the surface to be sanded. Move it in the direction of the grain, exerting gentle pressure on the tool. (Remember, the tools described here are to be used to put a finish surface on a workpiece, not to give it a basic shape, so don't put too much pressure on them). Keep the base of the sander flush to the surface being sanded at all times, to avoid creating undulations in the surface.